1. Field of the Invention
The invention relates to a device for protecting selective cutting machines against overload, in which the selective cutting machine has cutting tools, in particular cutting rollers, rotatably supported on a pivotable cantilevered arm, and the cutting tools and the cutting arm are connected to separate drive mechanisms.
2. Prior Art
Selective cutting machines, having cantilevered arms that are pivotable about at least two axes, as a rule have either cutting heads or cutting rollers. In the case of cutting heads, such cutting heads are usually designed for a particular pivoting speed of the cutting arm, so as to obtain a correct cutting pattern for a predetermined pressure resistance of a test cube on the part of the rock to be worked. Any deviation in pivoting speed from the pivoting speed for which the arrangement of cutters on the cutting head was designed can lead to severe vibrations, which can lead to the breakage of parts and especially of the cutters. Such impermissible vibrations have been detected in accordance with the proposal in German Patent Disclosure DE 33 43 372 A1 by a vibration sensor, and its signals have been evaluated and used for closed-loop control of the drive mechanism of the cutting speed.
However, above all in roller cutting machines, impermissible stresses and overloads are of particular importance, and such stresses cannot be detected correctly by vibration sensors. In roller cutting machines, a relatively wide roller is rotatably supported on the free end of the cutting arm; on pivoting of the cantilevered arm or cutting arm about a substantially vertical axis, the cutting forces act on the cutting or cantilevered arm via a long lever arm. When materials are being worked, it often happens that regions of less hardness or toughness or that are highly brittle are penetrated, and in these regions even large pivoting angles of the cutting arm have still not led to excessive loads, because the material can readily be cut or broken. However, if in certain pivoted positions a higher pressure resistance to a test cube is exerted on material, the result is impermissible stresses, which can cause heavy damage to the machine.
For monitoring impermissible operating states in roller cutting machines, it has previously been proposed that monitoring devices be used, in which the temperature of the liquid circulation of the hydraulic winch, the temperature of certain bearing points, the temperature of the coolant circulation, or the pressure as well has been monitored. Such monitoring devices are described for instance in German Patent Disclosure DE-OS 29 17 054. In German Patent Disclosures DE-OS 31 00 116 and DE-OS 31 06 348, special cutting tools can be found in which a cutting characteristic of the particular rock or mineral can be sampled during cutting by the cutting tool. Such devices are relatively complicated and expensive, because they have to be disposed in the immediate vicinity of the cutters acted upon by the cutting pressure, and the bearing points for pivoting the cutters are subject to correspondingly high wear.